The same as if they said it was copied from stack overflow, or if it’s wrong; “I think there’s a problem here, it’s XYZ”. If your peer ignores you and you were wrong, it was their call to make. If you were right - take it to them or the manager depending on how many times it’s happened.
Don’t ever write this in a professional environment. It’s childish ant achieves nothing other than pissing off the person it’s targeted at and probably the manager who now has to deal with a shitty behaviour complaint.